1. Field of the Invention
This invention relates to a magnetic disc cartridge which comprises a magnetic disc contained in a casing, and more particularly to a shutter spring for urging a shutter member for opening and closing a magnetic head insertion opening of the casing and a structure for incorporating the shutter member.
2. Description of the Related Art
There has been known a magnetic disc, a so-called floppy disc, which comprises magnetic layers formed on opposite sides of a flexible disc base of a polyester sheet or the like. Information is recorded on the floppy disc by a magnetic head while rotating the floppy disc. Further there has been provided a magnetic disc cartridge comprising a thin casing and a magnetic disc contained in the casing. Such magnetic disc cartridges have been mainly used as recording media for computers because they are easy to handle and available at low cost.
The magnetic disc cartridge is provided with a magnetic head insertion opening for giving a magnetic head access to each side of the magnetic disc for recording and reproduction. The magnetic head insertion opening is provided with a shutter member which closes the opening when the magnetic disc cartridge is not in use in order to prevent dust and/or dirt from entering the casing.
The recording capacity of the magnetic disc has been increased through improvement in line recording density and track density by change in the recording system and/or improvement in the magnetic layer. In the magnetic disc cartridge of an increased recording capacity, it is necessary to improve the dust-proof performance of the casing in order to avoid adhesion of foreign matters as much as possible, thereby improving reliability of the magnetic disc cartridge.
When a magnetic disc cartridge larger than conventional magnetic disc cartridges in recording capacity is developed, a novel disc drive system for driving the magnetic disc cartridge having a larger recording capacity is provided. Such a novel disc drive system is generally arranged to be able to drive magnetic disc cartridges having smaller storage capacities as well as the newly developed larger capacity magnetic disc cartridge. Such capability of driving magnetic disc cartridges having smaller recording capacities as well as the newly developed larger capacity magnetic disc cartridge will be referred to as "downward compatibilities", hereinbelow.
In order to provide such downward compatibilities to the disc drive system for the larger capacity magnetic disc cartridge, the novel larger capacity magnetic disc cartridge must be substantially equal to the conventional magnetic disc cartridge in principal dimensions and structure.
Since the conventional small capacity magnetic disc cartridge is not so high in dust-proof performance, the larger capacity magnetic disc cartridge must be improved in dust-proof performance without essentially changing the principal dimensions and the structure in order to obtain the downward compatibilities.
An approach is to close a spindle hole when the magnetic disc cartridge is not in use, which hole is formed in the lower surface of the casing in order to give a spindle of a disc drive system access to a center core of the magnetic disc. The spindle hole is kept open even if the magnetic disc cartridge is stored, in the conventional magnetic disc cartridge where the dust-proof performance need not be so high. For example, the spindle hole can be closed by a shutter member for opening and closing the magnetic head insertion opening, as disclosed in our Japanese Patent Application 9(1997)-90620.
In this case, since the diameter of the spindle hole is larger than the width of the magnetic head insertion opening, the shutter member must be slid by a larger distance, which results in the necessity of increasing the stroke of a shutter spring for urging the shutter member to its closing position. In the conventional magnetic disc cartridge, a coiled torsion spring defined in JISB0103 is used. Attempts to increase the stroke of such a shutter spring gives rise to a problem that the overall size of the shutter spring has to become large in order to ensure a desired stroke and a space for permitting deformation of such a large spring has to be conserved.
Specifically, the conventional shutter spring of a coiled torsion spring comprises a coiled central portion of several turns and linear end portions extending in two directions from opposite ends of the central portion. One end portion of the shutter spring is fixed to the casing and the other end portion of the shutter spring is fixed to the shutter member. When the shutter member is opened or closed, the end portion fixed to the shutter member makes a linear motion and the coiled central portion makes a circular motion about the end portion fixed to the casing. In order to increase the stroke of such a shutter spring, the opposite end portions of the spring must be elongated. When the end portions are elongated, the path of the circular motion of the central portion and the path of the linear motion of the end portion are extended toward the axis of rotation of the magnetic disc to interfere with the magnetic disc.
The coiled torsion spring may be replaced with a shutter spring disclosed in Japanese Unexamined Patent Publication No. 9(1997)-73749. As shown in FIG. 12, the shutter spring a comprises a coiled portion b and a pair of arms c extending from opposite ends of the coiled portion b. In this shutter spring, the central coiled portion b undergoes deformation in bend but not in torsion unlike the conventional coiled torsion spring, and accordingly the repetitional durability is improved.
However since the shutter spring only differs from the coiled torsion spring in that the coiled portion is deformed not in torsion but in bend, the arms must be elongated in order to increase the stroke of the shutter spring. This point is the same for all the conventional coiled torsion springs.